The present application does not claim the benefit of any previously filed and copending nonprovisional applications (or international applications designating the United States of America) under 35 U.S.C. xc2xa7xc2xa7 120, 121 or 365(c).
The present invention was not developed either wholly or partially under any federally sponsored research and development program or grant.
1. Technical Field
The present invention relates to an improved blowdown control method for preventing the formation of scale within a boiler system.
2. Description of the Related Art
Successful and efficient operation of industrial steam boiler systems requires that the various chemical constituents of boiler water be controlled in a manner that will avoid or at least minimize both the formation of mineral scale or deposits on heat exchange surfaces and the corrosion of metal surfaces within the steam boiler system. While deposition tends to involve corrosion products generated within the boiler system, scale is more commonly results from the precipitation of scale forming minerals, typically salts of calcium and/or magnesium introduced into the boiler systems with the makeup water.
Even with the best makeup water pretreatment, scale and deposition control chemistry is required to control residual amounts traces of scale forming minerals and any corrosion products formed within the boiler system.
Scale control products may be broadly classified as precipitating or non-precipitating depending on the manner in which they act to prevent scale formation. Precipitating products commonly rely on carbonate or phosphate chemistry to precipitate the hardness minerals and form a fine xe2x80x9cmud.xe2x80x9d These fine precipitates are typically maintained in suspension through the use of one or more polymer dispersants and are removed along with a portion of the boiler water during the boiler blowdown process. Non-precipitating products, however, are more commonly based on one or more sequestrants or chelant chemistries that react with scale forming minerals to form a soluble compound. As with the precipitating products, a portion of the dissolved compound are removed along with a portion of the boiler water during the boiler blowdown process, thus preventing scale formation.
A fundamental requirement of boiler system operation is the need to maintain the concentration of all hardness or scale forming minerals and their related compounds at levels below their solubility limits to prevent scale formation. As noted above, a variety of industrial boiler water scale and deposit inhibitors are helpful in this process. However, while boiler water treatment chemicals can be used to increase boiler system tolerance to scale forming minerals, chemical treatments alone are inadequate to prevent scale formation at higher boiler water concentration levels. Therefore, in addition to boiler water treatment chemicals, a volume of the concentrated boiler water must be periodically removed from the boiler and replaced with a generally corresponding volume of less concentrated feed water during operation with a blowdown process. This is referred to as boiler blowdown.
Blowdown processes can be broadly classified as either a manual blowdown process or an automatic blowdown process. As the term implies, manual blowdown is the periodic removal of boiler water by a person, typically the boiler operator, who opens one or more valves on the boiler system to initiate the blowdown process and then closes the valve(s) to terminate the blowdown process. In connection with the manual blowdown process, the boiler operator may also run chemical or other tests on boiler water samples to determine when to initiate the blowdown and/or to determine if a sufficient quantity of the concentrated boiler water was removed. Alternatively, the boiler operator may conduct the manual blowdown according to a schedule providing prescribed intervals and/or durations for the blowdown process. This may include intermittent xe2x80x98bottom blowdownsxe2x80x99 or a blowdown rate control valve setting (continuous blowdown) to try to maintain a set point for a desired boiler water dissolved solids level.
Automatic blowdown processes, on the other hand, utilize some form of instrumentation for measuring one or more properties of the boiler system that generally correspond to the concentration of scale forming minerals in the boiler water. When the measured parameter(s), such as electrical conductivity, reach a predetermined level, the automatic blowdown system will open one or more valves to initiate the blowdown process and/or introduce feed water into the boiler system. The automatic system will typically continue the blowdown process until the measured parameter(s) indicate that the scale forming mineral concentration within the boiler water has been reduced to an acceptable level and then terminate the blowdown process.
Although the automatic blowdown processes tend to represent an improvement over manual blowdown processes, the use of such processes still tend to result in periods of the boiler operating cycle during which the boiler water is either over-concentrated, increasing the likelihood of scale formation, or under-concentrated, reducing boiler efficiency and increasing the consumption of expensive boiler treatment chemicals. Further, the sensors used for measuring the selected parameters typically require increased maintenance and cost associated with sensor replacement, calibration, and/or cleaning. Some types of sensors, in particular those used to measure conductivity, are themselves sensitive to thermal shock and eventually can lead to an underreporting of the boiler water conductivity, further delaying the blowdown cycle and increasing the potential for scale formation.
The present invention provides a method for controlling boiler water concentration in boiler systems that utilize bimodal feedwater pumps that have only an ON mode and an OFF mode capable of providing feedwater to the boiler at a substantially constant flow rate in the ON mode. The present method is generally not applicable, however, to boiler systems that utilize boiler feedwater pumps that incorporate proportional feedwater flow control valves that regulate the feedwater flow rate within a given range.
In those boiler systems that utilize a bimodal boiler feedwater pump, the present invention utilizes the same signal or signals that control the boiler feedwater pump (i.e., turning it ON or OFF) to open or close a blowdown valve in synchronization with the boiler feedwater pump. When the boiler feedwater pump is turned ON, the blowdown valve is opened and boiler water is directed to a rate control valve that fixes the flow rate of the blowdown water. Using the pump curve data for the boiler feedwater pump, the flow capacity of the rate control valve is set to a predetermined percentage of the feedwater flow rate produced when the boiler feedwater pump is ON under the prevailing boiler conditions. This synchronized operation of the boiler feedwater pump and the blowdown valve assures that a fixed, proportional volume of concentrated boiler water is removed from the boiler whenever feedwater is entering the boiler and thereby maintain the scale forming minerals within a desired concentration range.